docs: def get_cell_boundary_map(ds):
      '''
      Returns a dictionary mapping a variable to its boundary variable. The
      returned dictionary maps a string variable name to the name of the boundary
      variable.
  
      :param netCDF4.Dataset nc: netCDF dataset
      '''
      boundary_map = {}
      for variable in ds.get_variables_by_attributes(bounds=lambda x: x is not None):
          if variable.bounds in ds.variables:
              boundary_map[variable.name] = variable.bounds
      return boundary_map